Apparatus for producing an interference signal at a selected location

ABSTRACT

An electrotherapeutic apparatus for producing a beat or interference frequency at a selected body location comprises two pairs of electrodes connected to the body. Alternating current is supplied to each pair of electrodes from the two outputs of an oscillator, with the current paths between the electrodes of each pair crossing each other at the selected body location. A phase shifter rhythmically changes the phase of the current in one of the current paths.

July 22, 1975 APPARATUS FOR PRODUCING AN INTERFERENCE SIGNAL AT ASELECTED LOCATION FOREIGN PATENTS OR APPLICATIONS 87l,672 6/1961 UnitedKingdomunn IZRMZU 467.502 ("i/I937 United Kingdom v. l28/42l) [76]Inventor: Hans Rodler, Karntnerstrasse l6l.

A'8O53 Grazr Austria Primary liruminerWilliam E. Kamm 22 Fihid: Sept 201973 Attorney, Ageiu, 0r Firm-Kurt Kelman [2 [1 App], No.1 399,067

Related U.S. Application Data ABSTRACT [63] (j ti tj i t f 5 No 7 9 Sam7 An electrotherapcutic apparatus for producing a heat I971. abandoned.or interference frequency at a selected body location comprises twopairs of electrodes connected to the [52] U.S. Cl 128/422; 128/420 body.Alternating current is supplied to each pair of [5 l] Int. Cl A6I|i 1/36electrodes from the two outputs of an oscillator, with [58] Field ofSearchWW. l28/4l9 R. 420, 421,422, the current paths between theelectrodes of each pair l28/423 crossing each other at the selected bodylocation. A phase shifter rhythmically changes the phase of the [56]References Cited current in one of the current paths.

D UNITE STATES PATENTS 9 Claims, 15 Drawing Figures 2.6221101 [2/1952Ncmec [28/420 1 b OSLI'LLATOR AMPUF/He 20 O arms/1mm 6 7 (incl/1rMMFRFGUEIVCY OSCILLATD nut Ti VIPRA TOR Hum V/B RA PATENTEmmzz ms3.895539 sum 1 HUMAN BUD) ELECT RUDE Fig. 1

PATENTEIJJULPZ 1915 3,895,639

SHEET 3 1 b OSCILLAT R AMPLIFIER L 2 0 -6-O7 DIFFEREMMM I LON-FREQUEN YOSCILL Mum'- VI'FRA TOR 15 17 o/FfRflWIh cIlRCl/IT 1a Ham'- 1 V/BRA wk 91% 8 ANPL/FIER g 3 COIVVEKTER PATENTED JUL 2 2 ms SHEET APPARATUS FORPRODUCING AN INTERFERENCE SIGNAL AT A SELECTED LOCATION This is acontinuationin-part application of my copending application Ser. No.178,159, filed Sept. 7, 1971, now abandoned.

The present invention relates to improvements in apparatus for producinga beat or heterodyne frequency at a selected location of a body, and isparticularly useful in electrotherapy for the human body.

in known apparatus of this type, two pairs of electrodes are connectedor applied to the body. Independent oscillating means supplies each pairof electrodes with alternating current, and the current paths betweenthe electrodes of each pair cross each other at the selected bodylocation.

This type of electrotherapy has the particular advantage that a weakcurrent of relatively high frequency is transmitted between the skin andthe applied electrodes, which does not irritate the skin, while arelatively strong interference current of low frequency is produced atthe intersection of the two current paths, due to the superposition ofone current on the other and the corresponding frequency differencebetween the two currents. Furthermore, the selected location may beaccurately determined by a suitable arrangement of the electrodes, andthis location may be in regions deep in the body and remote from theskin. Accordingly, this type of electrotherapy has been successfullyused to relieve pain, to exercise muscles, to treat joint diseases orneuralgia, to induce sleep, to improve blood circulation, and toalleviate inflammations.

1n known apparatus of this general type, two separate and independentoscillators have been used to supply the oscillations to the electrodes.the difference be tween the frequencies of the two oscillators beingvery small, i.e. within the range of about 0.5 to cycles per second(cps). In the usual oscillator frequency of 5000 cps, one cycle persecond corresponds to a tolerance of 0.02 percent, which is an almostunattainable accuracy since the two oscillators influence each other andtheir frequencies tend to become equal when the difference becomes toosmall. Since the frequency of at least one of the oscillators must becontrollable, it is impossible to attain such accuracy even with the useof quartz oscillators. Furthermore, these conventional electrotherapeutic machines produce only interference currents of sine waves.

It is the primary object of this invention to avoid the indicateddisadvantages of such apparatus and to provide a beat or interferencefrequency generator of simple construction and capable of producinginterference currents of any desired wave shape and frequency.

The above and other objects are accomplished in accordance with theinvention by providing a single oscillator having two outputsrespectively connected to a respective one of the pairs of electrodes,and a phase shifter for rhythmically changing the phase of the currentin one of the current paths. The oscillator outputs supply analternating current to each pair of electrodes, and the electrodes arearranged about a selected location so that the current paths of thepairs of electrodes intersect at this location. The phase shifter isarranged between one of the oscillator outputs and the pair ofelectrodes connected thereto for rhythmically changing the phase of thecurrent in one current path.

In one embodiment, the oscillator is a rectangular wave oscillator. Oneof the oscillator outputs is connected to a first pair of theelectrodes. and an output amplifier and if desired a wave converter arearranged between the one output and the first pair of electrodes. Amonostable multivibrator, whose pulse width is controlled by alow-frequency oscillator, is connected to the other output. a firstdifferentiating circuit being arranged between the other output and themultivibrator. A further differentiating circuit is connected to themultivibrator. and the other pair of electrodes is con nected to anotheroscillator controlled by the differentiated edge of the oscillationsproduced by the multivibrator. An output amplifier and if desired a waveconverter are arranged between the other oscillator and the other pairof electrodes.

Since a low-frequency oscillator controls the pulse width of themultivibrator, the generated pulse becomes narrower and wider incorrespondence to the rhythm of this oscillator. Therefore, the trailingedge of the oscillations produced with the multivibrator changes itsphase position in respect to the basic oscillation rhythmically in thefrequency of the controlling low-frequency oscillator. Bydifferentiating the trailing edge and controlling a further oscillator,which may also be a monostable multivibrator but may be a sine waveoscillator, too, the further oscillator may be made to generateoscillations rhythmically phaseshifted in respect to the firstoscillations generated in the rectangular wave oscillator. The phaseshift may be changed between 5 and 355. The frequencies of the twooutput currents are the same, the phase of the second output currentbeing variable in respect of the phase of the first output current. Whenthe two frequencies are brought into interference at the point ofintersection of the two current paths, an interference oscillation isproduced. With an alternating current source, the en veloping curve ofthe interference oscillation may take any shape or form, the wave shapebeing controlled by the low-frequency oscillator. If this oscillatorchanges the pulse width of the first monostable multivibratorrectangularly, a rectangular wave interference oscillation is generated.If the low-frequency oscillator generates a sine wave oscillation, theinterference oscillations is sineshaped, too.

According to one preferred feature of the present invention, the outputamplifiers have an output for alternating current and an output fordirect current. Also, the transformers may preferably be switched out ofthe operating circuit of the apparatus. This has the advantage that arectangular direct current pulse is received from the DC. output. Whenbrought into interference, this makes two variations possible, i.e. theDC. pulses may be brought into interference in opposite polarity, inwhich case the pulses cancel each other at the same phase and producealternating current at the opposite phase, or they may be brought intointerference at the same polarity, in which case a DC. pulse of the samefrequency as the basic frequency is produced at the same phase and a DC.pulse is produced at opposite phase as long as the phase shift is 180.Thus, it is possible to produce a direct current deep in the tissues ofthe body although pulses of relatively high frequency are generated.

In another embodiment of the invention, two or more monostablemultivibrators are connected to the rectangular wave oscillator via adifferentiating circuit, the

pulse width of the first monostable multivibrators being controlled by alow-frequency oscillator and each monostable multiyibrator having an endstage with a patient output. an output amplifier with a patient outputbeing additionally directly controlled by the rectangular oscillator.

This has the advantage that three or more output am plifiers and thusthree or more current streams for patients may be operatedsimultaneously. The first circuit receives directly the basic frequency.the second circuit receives the frequency from one monostablemultivibrator. and the third one receives it from the other monostablemultivibrator in the indicated operating circuit. By suitably adjustingthe basic pulse width and thus the phase position of the firstmonostable multivibrator. current amplification may be attained at theinterference point. i.e. the interference point may be located moreaccurately. The considerable advantage of such an arrangement resides inthe fact that the operating circuit comprises only onefrequency-determining oscillator. the frequency constant of thisoscillator not being critical. Therefore. the interference frequency maybe made as small as desired. Furthermore. by using DC. pulses of thesame amplitude and equidistantly paced. DC. voltage may be produced atthe interference point. A multiphased arrangement makes it possible toproject the interference point more accurately and to increase theenergy at the interference point in respect of the electrodes.

The above and other objects. advantages and fea tures of this inventionwill be better understood by reference to the following detaileddescription ofone preferred embodiment. taken in conjunction with theaccompanying drawing wherein FIG. I is a schematic view of a portion ofa human body to which two pairs of electrodes of an apparatus accordingto the invention are applied;

FIG. 2 is a circuit diagram illustrating a very simple circuit foroperating the apparatus;

FIG. 3 is a circuit diagram illustrating another operating circuit;

FIG. 4 is a detailed diagram of the operating circuit of FIG 3'.

FIG. 5 diagrammatically illustrates a detail of the cir cuit of FIG. 2;

FIG. 6 shows yet another operating circuit;

FIGS. 7 and 9 diagrammatically illustrate a detail of the circuit ofFIG. 6'.

FIG. 8 shows still another operating circuit;

FIG. 10 is a circuit diagram of a further embodiment of the operatingcircuit;

FIG. II shows a control circuit for the phase shifting means of FIG. I0;

FIG. 12 shows the arrangement of the electrode pairs in the operatingcircuit of FIG. 10; and

FIGS. I3 and 15 are circuit diagrams of three additional operatingcircuit embodiments.

Referring now to the drawing and first to FIG. I. there is shown theoscillator having two outputs constituted by pairs of terminals 2, 3 and4, S. The terminals 2, 3 of one output are connected to electrodes 6. 7of a first pair of electrodes. and terminals 4, 5 are connected toelectrodes 8, 9 of a second pair of electrodes. The two pairs ofelectrodes are connected or applied to a portion of the human body 10 tobe subjected to electrotherapy. When alternating currents whose phasesare shifted in respect of each other are supplied to the respectivepairs of electrodes from the output terminals, an interference currentis produced at the location of intersection of the current paths II and12 between the electrodes of the respective pairs. The desired location13 and depth of the location of intersection ofthe current paths isdetermined by the positions of the electrodes on the body. theelectrodes being quadrangularly arranged. as is well understood by thoseskilled in this art.

In the very simple operating circuit shown in FIG. 2, terminals 2. 3 ofone of the outputs of oscillator la is connected to the pair ofelectrodes 6, 7, see FIG. 1, by means of secondary winding 34 oftransformer 31 receiving the sine wave output of the oscillator. thesignal being amplified by amplifier 35 connected between the transformerwinding and electrodes 6, 7. The oscillator produces a sine wave ofabout 5 KHz (thousand cycles per second).

Phase shift circuit 32, part of phase shift means 32, 33. is connectedbetween the other oscillator output terminals 4, 5 and electrodes 8, 9of the other pair in accordance with the present invention. Phase shiftcircuit 32, part of phase shift means 32, 33, comprises secondarywinding 39 of transformer 31 from whose center tapping point aphase-shifted signal is transmitted to amplifier 35' connected betweenthe tapping point and electrodes 8, 9 so that this pair of electrodesreceives an amplified phase-shifted signal. It further comprisesfunction generator 33 connected to one end of winding 39 and controllingmotor 42 driving the adjustable element of potentiometer 38 foradjustment of the same, and condenser connected to the other end of winding 39. In this manner. the generator 33 electromechanically controlspotentiometer 38 and thus the cur rent phase supplied to electrodes 8, 9rhythmically.

The diagram of FIG. 5 shows the two parts of the voltage of transformerwinding 39 as vectors 36, 36, the part voltage of potentiometer 38 asvector 37 and the part voltage of condenser 40 as vector 43. The two vectors 36, 36 form the base of a right triangle whose two sides areconstituted by vectors 37 and 43. The output voltage vector is tappedfrom the center of the base and the point of intersection betweenvectors 37 and 43, which point lies in a circle about the center pointof the base. As vector 37 decreases. the output voltage becomes closerand closer to the voltage of vector 36. If the resistance ofpotentiometer 38 increases to decrease vector 43 and proportionallyincrease vector 37, the phase of the current is shifted in the oppositedirection.

If desired. the potentiometer and the condenser could be adjustedtogether. thus increasing the region of the phase variation.

The circuit diagram of FIG. 3 shows rectangular wave oscillator 14having a differentiating circuit 15 connected to one pair of its outputterminals. which generates an impulse from the leading edge of therectangular pulse of the oscillator. This pulse controls a monostablemultivibrator I6 whose pulse width is controlled by low frequencyoscillator 17. The differentiating circuit I8 connected to themultivibrator generates a new pulse from the trailing edge of theoscillations generated in the multivibrator. and this new pulse controlsa second monostable multivibrator 19. The pulse width of themultivibrator 19 is so adjusted that the pulse durations andinterruptions are of equal duration. The output amplifier and waveconverter 21 delivers the current from multivibrator 19 to electrodes 9.8 which are applied to the patient. Another output amplifier andconverter is connected to the other pair of output terminals ofoscillator 14 to deliver current to electrodes 6. 7 applied to thepatient. The two currents are phase-shifted in relation to each other bythe amount of the pulse width of the multivibrator 16.

FIG. 4 is a circuit diagram showing the operating circuit of the circuitcomponents of FIG. 3 in more detail. The circuit elements are well knownand. as readily available articles of commerce. require no furtherdescription.

The rectangular wave generator 1b is an astable multivibrator which isconnected to the differentiating circuit 15 by means of couplingtransformer 33. The differentiating circuit 15 is connected tomonostable multivibrator 16 by means of a coupling diode 22 to suppressthe pulses of the second portion of the pulses delivered bydifferentiating circuit 15.

The pulse width of the monostable multivibrator 16 is controlled by thelow-frequency oscillator 17 which is connected to multivibrator 16 viaamplifier 23, the oscillator 17 being a Wien bridge generator.Potentiometers 31 control the frequency of the Wien bridge generator 17.If desired. the wave shape of the oscillations generated by the Wienbridge may be adjusted in a known manner by potentiometers (not shown).

The rectangular pulses generated by multivibrator 16 are differentiatedin differentiating circuit 18 and are delivered to the monostablemultivibrator 19 via diode coupling 24 which suppresses the ascendingpulse portion. The latter multivihrator is so adjusted that the lengthsof the pulses and interruptions are equal. Therefore. the phase positionof the rectangular pulses varies rhythmically in respect of therectangular pulses generated directly by the astable multivibrator.Since the monostable multivibrator 19 is always controlled by theastable multivibrator. proper operation is assured even at stationaryphase shifting.

The rectangular oscillation is supplied from the multivibrator 19 to adriving stage 25 and amplified at output amplifier 21. By suitablydimensioning the switching elements of driving stage 25 and amplifier 21the rectangular pulses may be converted into sine wave pulses. Theoutput amplifier 21 is connected by means of a transformer coupling tothe first pair of terminals 4, 5.

The rectangular pulses of astable multivibrator 16 are delivered viacondenser 26 and a driving stage 27 to output amplifier 20. By suitabledimensioning the switching elements of the driving stage and theamplifier the rectangular pulses may be converted into sine wave pulses.The output amplifier 20 is connected by means of a transformer to thesecond pair of terminals 6, 7.

The two phase-shifted rectangular pulses are superimposed in atransformer coil 29. the generated interference current is amplified andsupplied to an indicator lamp 30 which shows the interference voltage.

The circuit is supplied by a current source 23 which includes aWheatstone bridge. condensers and a Zener diode 24 to maintain thevoltage constant.

The operating circuit of FIG. 6 is a modified version of that of FIG. 2.differing therefrom in that phase shifting circuits 32'. 32' areconnected between each output of sine wave oscillator la and eachelectrode pair. the phase shifted signals being amplified by amplifiers35, 35. Function generator 33' controls the phase shiftingpotentiometers in the phase shifting circuits in the same manner asdescribed in connection with FIG. 2. In this circuit arrangement. eachphase shifter needs to effectuate only a shift since this will encompassa phase shift region between 0 and for the two phase shifters.

FIGS. 7 and 9 show the phases ofa respective one of the phase shifters32'. 32' in the same manner as described in connection with FIG. 5.

FIG. 8 illustrates an operating circuit with electronic circuitelements. Since wave generator 1c is constituted by transistor 49,resonance circuit 44 and feed-back winding 45. Phase shifting means 32a,32a are connected to the secondary 39'. 39' of transformer 31' which isconnected to one output of the sine wave generator (compare FIG. 2).Each phase shifting circuit 32a again comprises a condenser 40' and anadjustable resistance constituted by field effect transistor 46.Function generator 33a rhythmically controls the transistors 46, 46 tochange the resistance thereof rhythmically. Resistances 47, 47 transmitthe biasing potential from terminal 18 to the transistors.

FIGS. 10 and 11 show an embodiment of the apparatus wherein four pairsof electrodes are arranged for application to a patient so as to providea multi-phase treatment for the patients body. As shown. sine waves fromoscillator 1d are transmitted to two pairs of electrodes 6, 7 and 8, 9as well as two additional pairs of electrodes 50. 50 and 51. 51, theoutput signals from the oscillator being phase shifted by respectivephase shifting circuits 32/) connected between the oscillator and eachof the four pairs of electrodes. Function generator 331) controls thephase shifting circuits so that each circuit produces a different phaseshift. as in the embodiments of FIGS. 6 and 8, the phase shifted signalsagain being amplified by amplifiers 35. 35. each of the four amplifierfeeding an amplified phase-shifted ignal to a respective one of the fourpairs of electrodes. The phase shifting circuits may be thoseillustrated in FIG. 8, for example.

FIG. 11 illustrates the phase shift control for the four circuits toobtain different phases in each circuit. The two integrated analogamplifiers S2, 53 form a triangular function generator. different directcurrent voltages being added to the control voltage of this generator atconnection 31 receiving these voltages from resistors 55, 56 andresistance controls 54. This produces triangular voltages at controlsignal output points 57. 58, 59, 60 which have added thereto differentdirect current voltages.

If desired, the phase shifting circuits may be differently dimensionedwhereby the initial output signal phases are different so that thefunction generator 33b may be in parallel circuit with the phaseshifting circuits 32b.

FIG. 12 shows the arrangement of the four pairs of electrodes, theelectrodes of each pair being substantially diametrically opposite eachother in respect of a common point of intersection where the currentdensity is multiplied so that an intensive electrical treatment isobtained in depth at a desired point within a patients body, theamplitudes of the current at the respective electrodes being uniform.

In the operating circuit of FIG. 13, the phase shifting of the outputsignal from oscillator 1e to the pair of electrodes 8, 9, via amplifier35, comprises a conven- 7 tional bucket brigade delay line device 63 andan im pulse generator whose frequency is controlled by functiongenerator 33c. The impulse generator consists es sentially of amultivibrator constituted by transistors 64 and 66. thefrequency-controlling resistances being formed by transistors 65 and 67which. in turn. are connected to generator 33a at 70. the generatorcontrolling the resistances and thus rhythmically changing the frequencyof the impulse generator 64. 66. As is known. bucket storage devicesstore analog signals. the storage time depending on the frequency oftheimpulse generator. In this manner. a phase change is produced betweenthe input signal at input 7i of the phase shift ing circuit and theoutput signal at output 62 thereof. this change being linearlyproportional to the frequency of the impulse generator. Thus, a rhythmicchange in the frequency of the impulse generator produces a rhythmicphase change.

ln the operating circuit of FIG. 14. the phase shifting is effected by atransductor or magnetic amplifier arrangement. Thus. the sine wavesignal coming from oscillator If is transmitted to a bridge circuitconsisting of three resistors 75 and the transductor or magneticamplifier means 73. 74. A second winding 76. 76 controlled by functiongenerator 33d pre-magnetizcs the inductors 73 and 74 differently so thatthe inductance of the inductors is rhythmically changed by generator$31!. This produces a phase-shifted output signal which is transmittedto amplifier 35' for electrodes 8. 9 while the original signal istransmitted directly from oscillator if to amplifier 35 for electrodes6. '7. The vector diagram of this circuit is similar to that of FIG. 5.

Finally. FIG. 15 shows a purely electromechanical phase shifting means.In this embodiment. the phase of the output signal from oscillator lg toelectrodes 8. 9 is shifted by an arrangement equivalent to a threephasemotor. the stator having three windings 77. 78. 79 which receive theoutput signal from sine wave generator lg. the third phase winding 78receiving the signal from the generator via condenser 81. A fourthwinding 80 is rotatably mounted in the rotor space and produces aphase-shifted output signal which is transmitted to amplifier 35' ofelectrodes 8, 9. The phase de pends on the angular position of coil 80and this may be rhythmically changed by motor 330 driving the coil. Ofcourse, the coil may also be rotated by an electronic 3-phase sine wavegenerator. the principle of operation being the provision of a rotarycoil within the field provided by surrounding three surrounding coils.Thus. the same voltage is induced in the fourth. rotary coil 80 in eachangular position thereof. Only the phase of the voltage is changed independence on this angular position.

It will thus be appreciated that a variety of phase shifting means maybe devised by those skilled in the art and. depending thereon. theoscillator providing alternating current to the pairs of electrodes maygenerate rectangular or sine waves. What is essential is that the phaseofthe current receiving from the osillator by one pair of electrodes isshifted in respect to that of the other pair of electrodes.

While the invention is particularly useful in electrotherapy. it may beapplied whenever it is desired to produce a beat or heterodynefrequency. For instance. the apparatus may be used to heat or meltmetallic work pieces at selected locations. particularly in theirinterior lt may also be useful in signal transmissions. in

which case the stable and phase-modulated oscillations may betransmitted over two independent transmission paths and then broughtinto interference in the receiver. In this manner. the modulation valueis available in the receiver as amplitudemodulated value so that themodulation value may be reconstituted by simplc demodulation anddisturbances in the transmission path may be eliminated at the receiverby limiting the amplitude.

What is claimed is:

l. Apparatus for producing an interference signal at a selected locationcomprising. in combination. oscillator means for furnishing anoscillator output signal hav ing a determined frequency and a referencephase. phase shift means connected to said oscillator means cyclicallyvarying the phase of said oscillator output signal. thereby furnishing aphase-shifted oscillator output signal. first electrode means connectedto said oscillator means for creating a first current having saiddetermined frequency at said selected location in response to saidoscillator output signal; and second electrode means connected to saidphase shift means for creating a second current having said determinedfrequency and a phase varying cyclically with respect to the phase ofsaid first current at said selected location in response to saidphase-shifted oscillator output signal. whereby interference betweensaid first and second currents creates said interference signal at saidselected location.

2. Apparatus as set forth in claim 1. wherein said oscillator meanscomprise a sine wave oscillator having a first and second output eachfor furnishing said oscilla tor output signal; and wherein said phaseshift means comprise a first phase shift circuit including a capacitorand a variable resistor connected to said second output. and means forcyclically varying the resistance of said variable resistor.

3. Apparatus as set forth in claim 2. wherein said first and secondelectrode means respectively comprise a first and second amplifier eachhaving an output. and a first and second pair of electrodes respectivelyconnected to said output of said first and second amplifier.

4. Apparatus as set forth in claim 3. wherein said phase shift meansfurther comprise an additional phase shift circuit having a capacitorand a variable resistor interconnected between said oscillator outputand said first electrode means, and means for cyclically varying theresistance of said variable resistor in said additional phase shiftcircuit in the direction opposite to the variation of resistance of saidvariable resistor in said first phase shift circuit.

5. Apparatus as set forth in claim 1, wherein said oscillator meanscomprise rectangular wave generator means for furnishing a rectangularwave having leading edges each indicative of the start of a cycle; andwherein said phase-shift means comprise delay means connected to saidrectangular wave generator means for furnishing a trigger signal after avariable time delay following each of said leading edges. and secondwave generator means connected to said time delay means for furnishing acycle of a second wave in response to each of said trigger signals.whereby said second wave has the same frequency but a varying phaseshift relative to said rectangular wave.

6. Apparatus as set forth in claim 5. wherein said second wave generatormeans comprise pulse furnishing means for furnishing a pulse in responseto each of said trigger signals.

7. Apparatus as set forth in claim 6, wherein said time delay meanscomprise first differentiating circuit means connected to saidrectangular wave generator means for differentiating said rectangularwave and furnishing first trigger signals. each indicative of one ofsaid leading edges; first monostable multivibrator means having atrigger input connected to said first differentiating circuit means anda control input. for furnishing a delay pulse having a pulse widthvarying as a function of the amplitude of a control signal applied atsaid control input in response to each of said first trigger signals;and low frequency oscillator means for furnishing a low frequencycontrol signal to said control input of said first monostablemultivibrator means, whereby each of said delay pulses has a trailingedge occuring at a varying time delay with respect to said leading edgesof said first rectangular wave; second differentiating circuit meansconnected to said first multivibrator means for differentiating saiddelay pulses and furnishing second trigger signals in response to saidtrailing edges of said delay pulses; and wherein said pulse furnishingmeans comprise second monostable multivibrator means having a triggerinput connected to said second differentiating circuit means forfurnishing a pulse having a determined pulse width in response to eachof said second trigger signals.

8. Apparatus as set forth in claim I, wherein said phase shift meanscomprise bridge circuit means having input terminals connected to saidoscillator means and output terminals connected to said second electrodemeans. and magnetic amplifier means having output windings connected inan arm of said bridge circuit and having input windings. and meanscoupled to said input winding for applying a cyclically varying currentthereto. thereby cyclically varying the inductance of said outputwindings and the phase of the signal at said output terminals of saidbridge circuit.

9. Apparatus as set forth in claim I, wherein said phase shift meanscomprise a three phase stator connected to said oscillator means, arotor connected to said second electrode means, and means forcontinuously rotating said rotor relative to said stator.

* I( I II!

1. Apparatus for producing an interference signal at a selected locationcomprising, in combination, oscillator means for furnishing anoscillator output signal having a determined frequency and a referencephase; phase shift means connected to said oscillator means cyclicallyvarying the phase of said oscillator output signal, thereby furnishing aphase-shifted oscillator output signal; first electrode means connectedto said oscillator means for creating a first current having saiddetermined frequency at said selected location in response to saidoscillator output signal; and second electrode means connected to saidphase shift means for creating a second current having said determinedfrequency and a phase varying cyclically with respect to the phase ofsaid first current at said selected location in response to saidphase-shifted oscillator output signal, whereby interference betweensaid first and second currents creates said interference signal at saidselected location.
 2. Apparatus as set forth in claim 1, wherein saidoscillator means comprise a sine wave oscillator having a first andsecond output each for furnishing said oscillator output signal; andwherein said phase shift means comprise a first phase shift circuitincluding a capacitor and a variable resistor connected to said secondoutput, and means for cyclically varying the resistance of said variableresistor.
 3. Apparatus as set forth in claim 2, wherein said first andsecond electrode means respectively comprise a first and secondamplifier each having an output, and a first and second pair ofelectrodes respectively connected to said output of said first andsecond amplifier.
 4. Apparatus as set forth in claim 3, wherein saidphase shift means further comprise an additional phase shift circuithaving a capacitor and a variable resistor interconnected between saidoscillator output and said first electrode means, and means forcyclically varying the resistance of said variable resistor in saidadditional phase shift circuit in the direction opposite to thevariation of resistance of said variable resistor in said first phaseshift circuit.
 5. Apparatus as set forth in claim 1, wherein saiDoscillator means comprise rectangular wave generator means forfurnishing a rectangular wave having leading edges each indicative ofthe start of a cycle; and wherein said phase-shift means comprise delaymeans connected to said rectangular wave generator means for furnishinga trigger signal after a variable time delay following each of saidleading edges, and second wave generator means connected to said timedelay means for furnishing a cycle of a second wave in response to eachof said trigger signals, whereby said second wave has the same frequencybut a varying phase shift relative to said rectangular wave. 6.Apparatus as set forth in claim 5, wherein said second wave generatormeans comprise pulse furnishing means for furnishing a pulse in responseto each of said trigger signals.
 7. Apparatus as set forth in claim 6,wherein said time delay means comprise first differentiating circuitmeans connected to said rectangular wave generator means fordifferentiating said rectangular wave and furnishing first triggersignals, each indicative of one of said leading edges; first monostablemultivibrator means having a trigger input connected to said firstdifferentiating circuit means and a control input, for furnishing adelay pulse having a pulse width varying as a function of the amplitudeof a control signal applied at said control input in response to each ofsaid first trigger signals; and low frequency oscillator means forfurnishing a low frequency control signal to said control input of saidfirst monostable multivibrator means, whereby each of said delay pulseshas a trailing edge occuring at a varying time delay with respect tosaid leading edges of said first rectangular wave; seconddifferentiating circuit means connected to said first multivibratormeans for differentiating said delay pulses and furnishing secondtrigger signals in response to said trailing edges of said delay pulses;and wherein said pulse furnishing means comprise second monostablemultivibrator means having a trigger input connected to said seconddifferentiating circuit means, for furnishing a pulse having adetermined pulse width in response to each of said second triggersignals.
 8. Apparatus as set forth in claim 1, wherein said phase shiftmeans comprise bridge circuit means having input terminals connected tosaid oscillator means and output terminals connected to said secondelectrode means, and magnetic amplifier means having output windingsconnected in an arm of said bridge circuit and having input windings,and means coupled to said input winding for applying a cyclicallyvarying current thereto, thereby cyclically varying the inductance ofsaid output windings and the phase of the signal at said outputterminals of said bridge circuit.
 9. Apparatus as set forth in claim 1,wherein said phase shift means comprise a three phase stator connectedto said oscillator means, a rotor connected to said second electrodemeans, and means for continuously rotating said rotor relative to saidstator.